As part of the Ansys policy, employees cannot debug UDFs on the forum. However, I would suggest you to have a look at the following things.
For the macro C_UDSI, if you try to use the macro before you have specified the user-defined scalars in your Ansys Fluent model then an error will result.
3.2. Data Access Macros (ansys.com) -> Check section 220.127.116.11
Please also have a look at the definition of the macro C_UDMI and how the arguments are defined at the link below.
3.2. Data Access Macros (ansys.com) -> Check section 18.104.22.168
Additionally, this is the link from the Ansys manual for defining the source term. Make sure that you have correctly defined the dS array with respect to the dependent variable.
2.3. Model-Specific DEFINE Macros (ansys.com) -> Check section 2.3.43
Lastly, these are the time dependent macros which you can use to access variables for previous timesteps.
3.5. Time-Dependent Macros (ansys.com)
If you are not able to access the link, please refer to this forum discussion: Using Help with links (ansys.com)
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